1. Field of the Invention
The invention relates generally to chip carriers, and more specifically, to a multi-chip land grid array carrier.
2. Description of Related Art
Computer processors include various cache memories, including memory caches and disk caches. A memory cache is a portion of memory made of high-speed static random access memory (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for main memory. Memory caching is effective because most programs access the same data or instructions over and over. By keeping as much of this information as possible in SRAM, the computer avoids accessing the slower DRAM.
Some memory caches are built into the architecture of microprocessors. Such internal caches are often called primary, or Level 1 (L1) caches. Many computers also come with external cache memory, called Level 2 (L2) caches. The L2 cache is coupled to a dedicated bus, sometimes referred to as a "backside bus." Like L1 caches, L2 caches are composed of SRAM but they are typically much larger. The L2 cache improves system-level performance by improving the processor's memory read and write performance, as well as decreasing the system bus utilization. The large L2 cache results in less processor read requirements to main memory, thereby reducing the number of times the processor needs to access the system bus.
For example, the Intel.RTM. Pentium.RTM. Pro processor package includes the microprocessor chip and an L2 cache die packaged in a single package. The microprocessor chip and the L2 cache memory die are both mounted in a dual-cavity microprocessor package. The microprocessor package may then be mounted on a system motherboard. The tight coupling of the microprocessor chip and the L2 cache improves system performance and efficiency. The Pentium.RTM. Pro processor architecture is described in the Intel Architecture Software Developer's Manual, Volume 1: Basic Architecture, 1996/1997, available from Intel.RTM. Corporation, and in Pentium.RTM. Pro Processor System Architecture, Mindshare, Inc., 1997, both of which are incorporated by reference herein in their entirety.
While cache devices are often implemented using multiple memory chips, a design such as the Pentium.RTM. Pro L2 cache comprises a single die. The size of the L2 cache varies according to various models of the Pentium.RTM. Pro available. For example, the processor may be implemented with 256 KB, 512 KB, 1MB, etc. of L2 cache capacity. Manufacturing the single, large memory die for the L2 cache may be difficult and expensive. Defects in a single-die L2 cache may not be discoverable until after the processor and L2 cache die are assembled into their shared package. If a defect is found in the L2 cache after it is assembled into the microprocessor package, the entire package often must be scrapped. Thus, it may be desirable to implement the L2 cache in a manner that allows additional flexibility and simplifies manufacturing and testing.
Mounting the cache memory chips directly to a motherboard, as in many prior art cache implementations, greatly reduces performance. With cache memory implemented on the motherboard, each semiconductor die comprising the memory device is typically mounted in a conventional single-die package. The single-die packages are then soldered directly to the motherboard or mounted in sockets. The speed at which the cache runs is significantly slower when implemented on the motherboard.
In a compromise solution, single-die memory devices are coupled to a daughterboard along with the microprocessor. The daughterboard is then plugged into the motherboard. While this cache implementation improves performance over directly mounting the cache memory packages on the motherboard, it requires a larger footprint since the cache comprises several conventional single-die packages. Moreover, the daughterboard implementation still operates at a significantly slower speed than an integrated L2 cache. In one prior art daughterboard L2 cache implementation, the L2 cache operates at only half the speed of the processor.
Rather than using several single-die memory devices for an L2 cache, several semiconductor dies could be directly mounted in a processor package using conventional methods, such as controlled collapse chip connection (C4). This also has drawbacks. For example, the memory device semiconductor die may not be tested until mounted along with the microprocessor chip. If a single memory chip is defective, the entire microprocessor package must be scrapped, as removing and replacing a single semiconductor die is, at best, very difficult if not impossible.
The present invention addresses some of the above mentioned and other problems of the prior art.